Researchers commonly use genetically engineered mice to study cancer, but the animal disease differs slightly from the human one. So researchers have sought to transplant human breast tissue into mice to make a better model. Now Robert Weinberg of the Whitehead Institute for Biomedical Research in Boston and his team have succeeded. [...]

The cells grow into human-like breast tissues, complete with milk ducts. Unlike human breasts, however, the mice's growths sit flush to the chest. Humans are unusual in this respect, says Daniel Medina who studies breast cancer at Baylor College of Medicine at Houston, Texas: "In few other species are breasts pendulous."

The human-breasted mice also develop cancer in much the same way as humans. Scientists think breast cancer starts when one epithelial cell gets a mutation in its DNA and starts dividing wildly into a tumour.